MSCs interact with endothelium in vivo. MSCs (passages 6-9) were fluorescence-marked with PKH-26 as described in “Materials and methods,” injected intra-arterially into wild-type or P-selectin^–/– mice and followed microscopically in an ear window. (A) Representative views of a fluorescing cell, rolling on a microvessel. Numbers represent time points in seconds after start of analysis. (B) The rolling fraction of all cells in murine skin postcapillary venules was calculated after video microscopic analysis in and MSCs. Values are means ± SD from 9 vessels/3 animals analyzed in wild-type mice and 6 vessels/2 animals analyzed in P-selectin–deficient mice.